The present invention relates generally to fluorescent multi-layer polymeric articles having an ultraviolet light screening layer to protect the fluorescence color stability of a fluorescent layer. More particularly, the invention is directed to articles, preferably comprising a plurality of retroreflective elements, in which a fluorescent polymeric layer is protected by a polymeric ultraviolet light screening layer fabricated from a U.V. light absorbing polymer, or a polymer capable of re-arrangement to a U.V. light absorbing polymer.
Retroreflective sheeting is widely used for traffic and roadway safety signs. Such sheeting is typically provided as a polymeric monolayer or multilayer sheeting material having thousands of retroreflective elements, such as microprismatic comer cubes or glass microspheres, that reflect incident light. It is well known to incorporate one or more fluorescent dyes into a retroreflective sheeting in order to enhance the visibility of articles such as road signs manufactured from such sheeting material. Fluorescent colors enhance visual contrast, which makes fluorescent colored materials more conspicuous than nonfluorescent materials. Unfortunately, most fluorescent colorants have poor ultraviolet light stability. In some cases, fading of fluorescent sheeting due to ultraviolet light exposure can occur within six months. The loss of fluorescence caused by ultraviolet light exposure dramatically shortens the useful life of fluorescent traffic and roadway signs. Accordingly, there is a need in the art to stabilize fluorescent colorants in plastics and to find a means to reduce the fading of fluorescent dyes in order to provide articles such as retroreflective road signs that can remain in service for significantly longer periods.
To enhance the outdoor durability of fluorescent retroreflective sheeting, an ultraviolet light screening layer is often used to protect the base fluorescent polymeric matrix layer from the effects of ultraviolet radiation. Traditionally, the U.V. light screening layer is made by incorporating U.V. light absorbing compounds into a transparent polymer matrix. Japan Kokai No. 2-16042, Application No. 63-165914 (Koshiji et al.) and U.S. Pat. No. 5,387,458 (Pavelka et al.) each disclose fluorescent articles consisting of an ultraviolet screen layer disposed in front of a fluorescent layer. According to these references, the screening layer contains substantial amounts of ultraviolet light absorbing compounds, which absorb a defined range of U.V. light (wavelengths from 290 to 400 nm).
Such prior art multilayer structures in which a U.V. light absorbent additive-treated screening layer is disposed in front of a layer containing a fluorescent dye can give rise to several difficulties. One problem is that the U.V. light absorbent additives incorporated into the U.V. light screening layer may leach out with time, because most U.V. light absorbing compounds are relatively small molecules and the U.V. light screening layer is typically quite thin. As a result of this phenomenon, the screening layer may lose its protective function, and the fluorescent colorants in the fluorescent layer will quickly fade and lose their fluorescence when exposed to ultraviolet light. A further problem with the U.V. light absorbent additive-treated screening layers is that U.V. light absorbing compounds present therein can diffuse or migrate into the fluorescent layer. If the U.V. light absorbing compound is not carefully selected, this diffusion can actually accelerate the fading of the fluorescent colorant even though the diffused compound is one that absorbs U.V. light. The problem of additive migration requires that a U.V. light absorbing additive incorporated into a screening layer be carefully matched to the fluorescent colorant so as to minimize any tendency of the migrating U.V. light absorber to affect the color and fluorescence of the articles. The implication that one may randomly select any U.V. absorber capable of blocking most of U.V. light below 400 nm wavelength (see, e.g., Japan Kokai No. 2-16042, Application No. 63-165914 (Koshiji et al.) and U.S. Pat. No. 5,387,458 (Pavelka et al)) fails to take into account the potential interaction between the U.V. absorber in the screening layer, and the fluorescent dye(s) present in the colored layer.
Based on the problems described above, there is a strong need in the art for polymeric articles in which an ultraviolet light screening layer can provide longer lasting U.V. light protection to a polymeric layer containing a fluorescent dye. Moreover, there is a need in the art of manufacturing of such articles to be able to select a fluorescent colorant without regard to the type of U.V. light absorbing material present in the screening layer, and vice versa.
In view of the foregoing, a general object of the present invention is to provide a polymeric multilayer article in which a polymeric ultraviolet light screening layer is arranged in protective laminar fashion (with or without intervening layers) with a fluorescent dye-containing layer in order to provide a much higher degree of fluorescence and color stability in the dye-containing layer than is currently afforded by conventional U.V. light screening layers comprising a polymer and a U.V. light absorbing additive.
Another object of the invention is to provide a polymeric multilayer retroreflective article in which an ultraviolet light screening layer is fabricated from a polymer capable of absorbing ultraviolet radiation such that the retroreflective article can be used to fabricate fluorescent, outdoor-weatherable products that have greater durability in terms of both color and fluorescence.
Yet another object of the invention is to provide a polymeric multilayer retroreflective article in which an ultraviolet light screening layer is disposed in protective relation to a fluorescent layer having cube comers formed on a surface thereof, and the ultraviolet light screening layer is fabricated from one or more U.V. light absorbing polymers or polymers capable of undergoing re-arrangement to an ultraviolet light absorbing polymer.
Still a further object of the invention is to provide a polymeric retroreflective article in which an ultraviolet light screening layer fabricated from a U.V. light absorbing polymer, or from a polymer capable of re-arrangement to a U.V. light absorbing polymer, can be attached to, or otherwise arranged in protective laminar configuration with, a polymeric layer comprising a fluorescent dye to reduce loss of fluorescence upon exposure of the article to ultraviolet radiation, even if no additional U.V. light absorbing additives and/or light stabilizers are used in the screening layer and/or the colored layer.
Yet another object of the invention is to provide retroreflective sheeting material in which an enhanced ultraviolet light screening layer, fabricated from a U.V. light absorbing polymer, or from a polymer capable of re-arrangement to a U.V. light absorbing polymer, can provide a remarkable degree of fluorescence protection to an underlying fluorescent layer, thereby extending the useful life of such retroreflective sheeting material significantly beyond that currently attainable using conventional screening layers containing U.V. light absorbing additives.
These and other objects will become apparent hereinafter to those skilled in the art.
The present invention results from our discovery that a U.V. light screening layer fabricated from a U.V. light absorbing polymer, and/or a polymer capable of undergoing re-arrangement to a U.V. light absorbing polymer, can provide a remarkable level of protection to the fluorescence and color durability of a polymeric layer containing a fluorescent colorant. The level of protection is superior to that afforded by U.V. light screening layers in which one or more conventional U.V. light absorbent additives (e.g., benzophenones and/or benzotriazoles, either with or without hindered amine light stabilizers, and the like) are merely added to a non-U.V. light absorbing polymeric resin. Although U.V. light absorbing resins have been known (e.g., polyarylates), it could not have been predicted that the use of such resins in a multilayer fluorescent retroreflective structure would afford such a surprising degree of U.V. protection compared to the U.V. additive-treated screening layers that have heretofore been used.
Accordingly, we have found that the objects stated above can be achieved in a multilayer, U.V. light protected article, preferably a sheeting material, comprising: (a) a polymeric layer comprising a fluorescent colorant; and (b) an ultraviolet light screening layer disposed over the colored layer and comprising either a U.V. light absorbing polymer, or a polymer which can undergo photo-Fries re-arrangement to a U.V. light absorbing polymer. More particularly, the polymeric resin used to fabricate the U.V. light screening layer comprises at least one polymer, or a mixture thereof, selected from the group consisting of (i) polymers having a polymeric backbone comprising the following repeating moiety A: 
wherein R is a non-interfering substituent and P is the remainder of the polymer, and whereby the polymers are able to absorb ultraviolet light; and (ii) polymers having a polymeric backbone comprising the following repeating moiety B: 
wherein R and P are as defined above; said moiety B being transformable to said moiety A by photo-Fries rearrangement, whereby said polymers comprising moiety B can undergo re-arrangement to ultraviolet light absorbing polymers comprising moiety A.
The term xe2x80x9cnon-interfering substituentxe2x80x9d is intended to denote substituents which do not prevent the above-defined moieties from exhibiting U.V. light absorbent properties, or from undergoing photo-Fries rearrangement to U.V. light absorbing moieties.
The screening layer may also comprise a mixture of the moiety A and moiety B polymers. Without intending to be bound to any particular theory, the enhanced color and fluorescence protection offered by screening layers of the present invention is believed to result, at least in part, from the repeating U.V. light absorbing moieties being present directly in the backbone of the screening layer polymer, as distinguished from separate U.V. light absorbing compounds admixed with polymer.
The invention is further directed to a retroreflective sheeting material comprising a plurality of retroreflective elements, a U.V. light screening layer fabricated from a polymeric resin comprising polyarylate, and a polymeric layer comprising a fluorescent dye, and, optionally a polyarylate-containing resin.
The multi-layer article of the instant invention finds particular utility in a retroreflective road sign comprising: (i) a polymeric layer comprising a fluorescent dye, and optionally a polyarylate, (ii) a polymeric U.V. screening layer disposed in front of said fluorescent layer and comprising polyarylate, and (iii) a plurality of retroreflective elements disposed such that incident light emanating from an automobile""s headlamps is retroreflected by the road sign back to the driver of the vehicle.
Preferred polymers for the screening layer are polyarylate, and blends of polyarylate with any one or more of the following: polycarbonate, poly(cyclohexanedimethanol terephthalate) (xe2x80x9cPCTxe2x80x9d), poly (cyclohexamedimethanol-co-ethylene terephthalate) (xe2x80x9cPETGxe2x80x9d), and poly(ethylene terephthalate) (xe2x80x9cPETxe2x80x9d). Although the polyarylate backbone contains a moiety B capable of undergoing photo-Fries re-arrangement to a benzophenone-type U.V. light absorbing moiety, the invention is also directed to polymers which have benzophenone-type moieties (see moiety A above) in their polymeric backbone and thus do not require re-arrangement to become U.V. light absorbent.
Numerous advantages are provided by the invention. For example, it is no longer necessary to select the fluorescent colorant in a fluorescent layer for compatibility with the type of U.V. light absorbing material used in the screening layer. It is believed that the U.V. light absorbent polymer layer as used in the structure of the present invention will provide excellent protection to virtually any fluorescent colorant present in the fluorescent layer. Another advantage is the ability to substantially reduce or eliminate the use of separate U.V. light absorbing additives, thereby reducing or eliminating the known problem of such additives leaching or migrating out of the screening layer. Yet another advantage of the invention is that, in a preferred embodiment in which polyarylate is present as the U.V. light absorbent polymer in the screening layer, the polymeric screening layer also provides a combination of excellent properties important in the fabrication of durable retroreflective sheeting material such as toughness, impact resistance, and chemical resistance.